Answer: The right answer is the A) It emphasizes the sadness of saying farewell.
Explanation: Just to elaborate a little bit on the answer, it can be added that the words in this poem should not be interpreted literally—darkness, for the speaker, is not a lack of light, but a lack of a human presence—, so option C should be discarded. Option B is not accurate, either, since the reference to the brain does not mean that she is sharing her personal thoughts. After having explained, in the two first stanzas, that it takes time to get used to the idea that a neighbor is about to say goodbye, but eventually one does it, in the third stanza the speaker emphasizes the emptiness and the sadness that one feels afterwards. The mind is in the dark for long days and nights, and nothing sparks within it.